Airlines want less weight, so they use less fuel, therefore reduce costs. Manufacturers Airbus and Boeing strive and compete with each other to achieve this. Pushing the boundaries results in problems, so both the 380 and 787 have come across snags.
So true. Also true (certainly when they go into service with us) is that they will be carrying
tonnes of duty-free goods that don't get sold, magazines that don't get read, bottles of wine/spirits that don't get drunk, etc. The overall weight saving between using off-the-shelf tech like NiMH vs. Li-ion with extra fire precautions must be pretty small in relation to the eventual usage pattern of the aircraft but has grounded it indefinitely.
I think most airlines prefer a slightly heavier aircraft that they can fly, rather than one that sits on the ground looking pretty...